How to animate a surface that only emits light when touched by object?
Set up your reactive surfaces as a Canvas for Dynamic Paint, and your contact surfaces as Brushes.

You can store the effects of the brush on the canvas either as vertex data or as a a UV-mapped image.

In this example, The steps were UV unwrapped, and a wetmap was baked as an image sequence, which was then used as the factor to mix between a diffuse and emissive shader in the steps' material. The wetmap was set to fade after about 40 frames. On the whole, the tools guide you through their use; I can't think of any major gotchas.

The Canvas (Steps) was set up like this:



The Brush like this:



I baked out the maps using the 'Bake Image Seqence' button in the 'Canvas' panel.

You can use the image sequence as a mask. something like this:



And this is the sort of result -



The method given here bakes the paint result to image-maps, on the assumption that you won't want a highly subdivided model in your architectural render. However, to preview the effects of the various settings, it would be better to make a highly subdivided test-bed canvas, (a plane will do) and set up a simple brush, say a sphere. Then set the 'Format' in the canvas to 'Vertex', and the mode to 'Paint'. Then by having the animation set to continuous looped playback in the timeline, and moving the brush with respect to the canvas, you can see the effects of settings live in the viewport.

I would suggest you want to select 'Wetmap' as the preview in the Output sub-panel, and play with 'Spread' and 'Shrink' effect speeds, and 'Drying' and 'Dissolve' times in the Advanced panel. Also adjust the initial 'Wetness' (load) of the brush. These will all interact to give a variety of organic feels to the animation.

Don't forget, either, that the wetmap can wind up being used to mask a variety of other effects in the material of your steps, including, for example, other textures, which may themselves be animated. The results can also be used to control particle systems.